Optical pumping of alkali vapors in gases has found increasingly diverse applications in science, medicine and military defense. In basic science, for example, measurements of the substructure of protons and neutrons were enabled by producing polarized nuclei of helium-3 by Spin Exchange Optical Pumping (SEOP). In diagnostic medicine, SEOP was applied to hyperpolarize the nuclei of both helium-3 and xenon-129 for uses in magnetic resonance imaging of pulmonary function. For both these applications, laser light tuned to the absorption wavelength of the D1 line in rubidium at 795 nm is circularly polarized and directed to illuminate mixed gases saturated with rubidium vapor in a magnetic field, causing the rubidium electron to become polarized. Through collisions with helium-3 or xenon-129, the angular momentum is transferred to the nuclei of these atoms, making them nuclear-polarized. These applications benefit from a new and growing class of lasers with selectable central wavelength and linewidth narrowed from their natural width of 3 nm to a narrowed level of 0.2 nm or less, with power ranging from hundreds of watts to a few kilowatts. Efforts are ongoing to further reduce the linewidth to approach the pressure broadened linewidth of the gas (around 0.01 nm).
Laser diodes pumping a gas saturated with alkali vapor may also be a very efficient method for creating a single-aperture diffraction-limited high-power (megawatt) laser beam for military defense applications. In a Diode Pumped Alkali Laser (DPAL), for example, a plurality of pump lasers illuminate a gas saturated with an alkali vapor at the D2 line, causing the atoms to populate the P3/2 second excited level. Collisions with gas atoms quench the P3/2 to the lower-lying P1/2 first excited level. A population inversion between this first-excited state and the ground state allows stimulated emission to occur, creating a lasing transition at the D1 line. This application would benefit from a bank of pump lasers with selectable central wavelength whose spectral width is narrowed close to the pressure broadened linewidth. Since the optimal operating pressure may be low, the amplifying medium may be long and narrow (e.g., perhaps a centimeter or less), and it may be desirable to pump the medium transversely, a pump system with a very high absorption cross section is desirable. Delivering all the pump power within a linewidth matching the pressure broadened linewidth of ˜0.01 nm or even narrower may be desired.
Diode laser sources, such as diode array bars, used in high-power diode laser array systems provide wavelengths at 795 nm but with a broad 3 nm wide spectral output linewidth. Existing technologies used to select the central wavelength and narrow the spectral output of high-power diode laser array systems incorporate elements, such as a volume Bragg gratings (VBGs) or planar diffraction gratings, in an external lasing cavity to allow preferential feedback of the preferred wavelength. The laser linewidths currently available from both of these diffractive technologies are significantly broader than the pressure broadened absorption linewidth of the vapor in the applications mentioned above.
Atomic line filters (ALFs) provide passbands of about 0.001 nm and have been used to improve the background rejection of conventionally filtered laser receivers. In general, ALFs make use of narrow, sharp features in the spectra of atomic vapors (e.g., alkali metal vapors) to provide ultra-narrow optical passbands. ALFs have been based on the Faraday effect (i.e., Faraday filters) by rotating polarized light when it passes through a resonant vapor medium in the direction of an applied magnetic field. ALFs have also been based on the Voigt effect (i.e., Voigt filters) by transforming linearly polarized light into elliptically polarized light, finally becoming linearly polarized along the orthogonal direction.
Atomic line filters, particularly Faraday filters, have also been used in the external cavity of a diode lasing element to create a single mode laser in a closed-locked loop locked to the central wavelength of an atomic line. A unique challenge when using ALFs in the external cavity of such a laser is to assure that one of the allowed modes lies at the center of the atomic transition line and to assure that all of the laser energy is concentrated in this single mode. In one example of a single mode laser with an ALF, using a short external cavity baseline assures that the wavelength separation between longitudinal modes is large compared with the optical transition width of the ALF, and then feedback is provided to shift the back mirror position and adjust the length of the baseline to maximize the optical transparency of the atomic line filter.